1. Technical Field
The present invention relates to a magnesium alloy sheet and a method of manufacturing the same. More particularly, the present invention relates to a magnesium alloy sheet having a high bendability that which is used for applications requiring cold working or warm working, including press forming, deep drawing, and bending.
2. Prior Art
Heretofore, several techniques associated with magnesium alloys have been disclosed by the prior art, including the Japanese Patent Provisional Publications JP A H02-57657, JP A H02-57658, JP A H06-81089, JP A H06-293944, JP A H07-188826, JP A 2001-200349, JP A 2001-294966, and JP A 2002-121657, for example.
However, such techniques of the prior art have involved a very difficult problem in the workability of magnesium alloy, as will be described in greater detail herein below.
(1) Since magnesium, as a simple substance or its alloy, takes on a hexagonal closest packed crystal structure, it does not provide adequate slip systems required for plastic processing and, particularly, its warm workability is remarkably low at temperatures of 200° C. or below. Therefore, when fabricating a molded product by press forming a magnesium alloy sheet, such a low workability of the magnesium alloy has been a principal factor lowering the working productivity remarkably.
For press forming a magnesium alloy sheet, it is necessary to heat a mold and its associated parts required for pressing at about 200° C. or higher temperatures, because cracks or other failures will occur at ordinary temperatures to render the working very difficult. As a result, the prior art method inevitably requires energy and equipment for heating the mold.
Further, even when warm working is to be done by raising the mold temperature above normal, it is difficult to increase the strain rate (working speed) beyond a certain limit due to surface cracking and other defects occurring under such conditions, and thus it has so far been necessary to use a strain rate lower than a certain level.
(2) Magnesium alloy sheets of the prior art tend to exhibit an inferiority in the bendability that affects most greatly their press formability or cold/hot press formability.
Among wrought magnesium alloys obtained by rolling, those alloys belonging to the ASTM AZ31, AZ61 and the like types are used as materials having the greatest versatility. Although aluminum or other elements contained in such magnesium alloys improve their strength to a certain extent, their ductility and toughness are adversely affected thereby. Generally, in metal materials, as the strength increases, their reduction of area (area reduction), elongation, bendability or deep drawability that are measures indicating their ductility and toughness will decrease.
Although the strength and toughness may be increased by adding alloy elements (or tempers) such as strontium and rare earth metals, it adds up to increase in manufacturing cost. Especially, any extra addition of alloy elements may cause a problem of their irremovability in the phase of recycling that should be promoted socially or industrially from now on, thus bringing about a factor that hinders the recyclability.
(3) Although it is possible to expect in general an improvement of magnesium alloys in toughness by controlling their grains minutely, there is a limit to such grain size refinement, and the bendability which is most important for press formability is not improved above a certain level by such a means as grain size refinement.
Accordingly, a primary object of the present invention is to provide a magnesium alloy sheet that combines an excellent bendability with a sufficient strength and a method of manufacturing such a sheet.